Mqtt client 'offline' callback and fix small issue in getMessage method

[kenny59]

My bot was not responding after (each day after 5-8 hours of running) a while and it turns out that I was getting 'offline' callback from mqtt client, which fb-chat-api is not subscribed to. I could not replicate it by unplugging any cables or disconnecting from internet, so it is most likely something that comes from Facebook itself. Bumping mqtt client was not neccessary in the end, but it should contain some under the hood fixes that should make it more reliable.

Also this contains a small fix that fixes getMessage when requesting a message that contains attachment.

[miruchigawa]

This is what I've been looking for all this time